For many years an effective form of preventing the entry of naturally magnetic material dust (i.e. dust from iron ore) into the ball bearings of belt conveyor rollers has been sought. Such contaminating material, entering the bearing and polluting them, consequently significantly reduces their useful life by wearing or locking. However, because of the great number of rollers in a conveyor system, the seals could not increase friction without significant inpact on the power required to drive the conveyor. Therefore, rubbing seals were inappropriate, and labyrinth seals, by themselves, tended to permit the pollutants to enter.